Maliks Muwatta Book 29, Hadith Number 18.

Section : Annulment of Marriage by Husband’s Vow to Refrain from Intercourse (Ila).

Yahya related to me from Malik from Nafi that Abdullah ibn Umar said, “When a man makes a vow to abstain from intercourse with his wife and four months have passed he must declare his intent and either he is divorced or he revokes his vow. Divorce does not occur until four months have passed and he continues to abstain.”

Yahya related to me from Malik from Ibn Shihab that Said al-Musayyab and Abu Bakr ibn Abd ar-Rahman said about a man who made a vow to abstain from intercourse with his wife, “If four months pass it is a divorce. The husband can go back to his wife as long as she is in her idda.”